This invention pertains to high strength, cellulosic-gel-containing kraft paper and to a process for making the same.
In a typical procedure, cellulosic gels are prepared by feeding a chemical papermaking pulp such as a Kraft pulp to a conventional hydrapulper in which it is disintegrated into a fibrous slurry having a consistency of from 1 to 10%, preferably from 6 to 8%.
The resulting slurry is fed to a battery of several conventional pulp refiners arranged in series. The flow through the sequence of refiners is throttled down by appropriate valving to give a pressurized dwell or residence time sufficient to exhaustively hydrate the feed material and produce a hydrated cellulosic gel product.
The use of cellulosic gels prepared in this manner as binders in the manufacture of wet process composition boards of various categories is illustrated in the following patents:
______________________________________ Roberts U.S. 3379608-9 Roberts 4173248 Roberts U.S. 4043862 Heritage Canada 449167 Roberts U.S. 4077833 Roberts U.S. 4087317 ______________________________________
However, cellulosic gels heretofore have not been applied successfully as efficient binders in the manufacture of Kraft papers since it appeared that the physical properties of the gels would prohibit their successful application to this end use.
Papermaking machines operate at very high speeds, for example, at speeds of as much as 3500 feet per minute. It is necessary that the furnish supplied to them drain quickly through the wire, i.e. in a matter of seconds.
Cellulosic gels are thick, viscous, sticky liquids. It would seem self-evident that to include a significant proportion of them in a papermaking furnish, in order to improve the strength of the paper product, would increase the drain time of the furnish to such a level as to make it unsuitable for use on high speed paper machines. Accordingly, heretofore the desired high strength of the paper products has been developed by refining the pulp to a high degree, and by using long fibered stock which inherently imparts high strength to the paper. This is undesirable, since prolonged refining requires expensive equipment the use of which is attended by high power costs. Long fibered stocks are comparatively expensive. Substantial economies could be achieved by reducing the refining time and using short fibered stocks, for example, short fibered hardwood stocks.
I have discovered that, surprisingly, the inclusion of a substantial proportion of cellulosic gel in Kraft papermaking furnishes does not materially increase their drain times on the Fourdrinier wire. Additionally, their inclusion materially increases the strength of the paper and reduces significantly the refining time which must be applied to the freshly cooked pulp.
Accordingly it is the general object of my invention to provide a process which will result in the production of Kraft papers of improved quality at significantly lower cost.
A further object of my invention is the provision of a process for the production of Kraft papers of improved strength, and having smoother, harder surfaces.
Another object of my invention is the provision of a process for making kraft papers which permits the inclusion of a significant amount of low cost, short fibered pulp such as hardwood pulp without adversely affecting the strength of the paper products.
Generally stated, my presently described process of making high strength kraft papers characterized by the above noted advantages comprises first mechanically refining kraft papermaking pulp to a freeness value of about CSF 500-800. Next, a fluid mixture is made of from about 90 to about 99% of the resulting refined pulp and from about 1 to about 10% of hydrated cellulosic gel binder.
The kraft papermaking pulp which is a primary component of the mixture comprises from about 40 to about 100% by weight of freshly prepared or "virgin" kraft pulp and from 0 to 60% by weight of pulp prepared from reclaimed or "secondary" lignocellulosic fibers, which may include a substantial proportion of short fibers.
The mixture is run into a felted sheet which is pressed and dried to form the final kraft paper product.